The present invention relates to a control for a continuously variable transmission for an automotive vehicle for quick downshifting to establish an engine brake running.
U.S. patent application Ser. No. 922,400 discloses a continuously variable transmission which has a shift control valve with a spool mechanically connected via a shift operating mechanism to a shift actuator in the form of a stepper motor. The stepper motor is supplied with a stepper motor drive signal by a control unit and rotatable to assume an operating position instructed by the control unit. Assuming now that an automotive vehicle is travelling with a small reduction ratio established in the continuously variable transmission, when the control unit determines a downshifting toward a relatively large reduction ratio, the stepper motor is caused to rotate at a predetermined speed in a downshift direction. This causes the shift operating mechanism to initially cause the shift control valve to increase an effective drain area through which the hydraulic fluid passes toward a drain port. As a result, a pressure in the hydraulic fluid applied to the driver pulley drops to allow the V-belt to move one of the driver pulley discs away from the other. This movement of the driver pulley disc causes the shift control valve to decrease the effective drain area. When the spreading force applied by the V-belt to the driver pulley discs establishes a new equilibrium state with a force generated by the hydraulic fluid pressure applied to the driver pulley, a new reduction ratio is established. With the same drain effective area, a time required for the continuously variable transmission downshifts to a new relatively large reduction ratio differs depending on whether a positive torque is applied to the driver pulley or a negative torque is applied to the driver pulley. The negative torque is applied to the driver pulley when a driver shifts a manual valve from a D (drive) range position to a L (low) range position demanding an engine brake running. With the negative torque applied, since the wedging force applied to the driver pulley discs by the V-belt is weak, the spreading movement of the driver pulley discs is slow accordingly. This causes a slow downshifting under this condition as compared to a downshifting effected with the positive torque applied to the driver pulley during a kick-down operation.
Thus, the automotive vehicle with this automatic transmission is slow in establishing engine brake running after the driver has moved the manual valve from the D range to L range, failing to provide an effective engine brake feeling possessed by the driver. In order to speed up this downshifting, one conceivable measure is to shorten a time required for the stepper motor to assume a desired or target operating position instructed by the control unit by increasing the speed at which the stepper motor rotates. However, if the speed at which the stepper motor is increased sufficiently to speed up the downshift for establishing an engine brake running, the V-belt slips relative to the driver pulley during downshifting for a kick-down operation. Thus, there is the limit to increasing the speed at which the stepper motor rotates. In the case of the above-mentioned known continuously variable transmission, the speed at which the stepper motor rotates is constant.
An object of the present invention is to provide a control for a continuously variable transmission for quick downshifting to establish an engine brake running of an automotive vehicle.
More particularly, an object of the present invention is to provide a ratio control for a continuously variable transmission whereby quick downshifting is effected without causing a slip of a V-belt relative to a driver pulley during downshifting for a kick-down operation.